1. Field of the Invention
The present invention relates to the field of direct AC-to-AC converter systems in general and more particularly to matrix converters, i.e., wherein the conversion is performed through a plurality of bilateral switches controlled for conduction according to a preselected switching sequence.
2. Description of the Prior Art
See for instance, U.S. Pat. Nos. 4,648,022, 4,833,588, 4,962,339, and 5,005,115, all to C. D. Schauder. These patents are hereby incorporated by reference.
Typically, a matrix converter or "forced commutated cycloconverter" uses nine bi-directional AC power switches to achieve AC-to-AC conversion.
Conceptually, the switches are arranged in three groups of three, each group being associated with a particular output line. Within such a group, the switches are operated so that the group output voltage consists of a sequence of samples of the input line voltages.
Generally, the matrix converter provides unrestricted frequency conversion, high quality input and output wave forms, and unity input displacement factor. It has the inherent capability to regenerate from a load to the AC mains supply and is particularly attractive because it does not require bulk energy storage components. In principle, a matrix converter switching at a high frequency can have a smaller physical size than other power converter types with the same capability.
With current matrix converter circuits, two rules must be followed. First, it is important that one, and only one, switch in a group should be closed at a time. Closing more than one switch in a group would short circuit the input lines. Second, at least one switch in a group should be closed at a time. Opening all switches in a group could interrupt the load current which is always supported by load circuit inductance. By opening all three switches in each group, a high voltage develops across the switches. To accommodate potentially high voltages which may develop as a result of forced commutation, matrix converter circuits typically include a voltage clamp.
One existing forced-commutation process that observes these switching rules steers the output load current from a closed switch, SW1, within a group to the next switch, SW2, in sequence within a group. The voltage across SW1 rapidly rises to a preselected level limited by a voltage clamp when SW1 is opened. The preselected level can exceed the maximum phase-to-phase source voltage. While the load current flows through the voltage clamp on SW1, a small delay, or underlap period, is allowed to expire before SW2 is turned on. A commutation loop current, flowing in the closed loop formed by SW1, SW2 and two phases of the power source, then rises approximately linearly from zero to the load current. When the commutation loop current equals the load current, the current through the SW1 voltage clamp reaches zero and the entire load current flows in SW2. In practice, the underlap period is partly intentional to account for imprecise detection of the recovery instant of SW1 and partly due to the turn-on delay of SW2.
During the commutation process, the load current is substantially constant, supported by the inductive load. When SW1 is turned off, the current through SW1 falls to zero and the current is diverted through a voltage clamp associated with SW1. The commutation loop current continues to flow solely through the voltage clamp during the underlap period until the next switch in the sequence, here SW2, is turned on. As a result, a substantial amount of power is absorbed by the voltage clamp every time switching occurs, from the time SW1 is turned off until SW2 has begun to carry the entire load current. This power is unavailable for work by the load, and is lost.
There is a need, therefore, for a matrix converter switch and commutating method which reduce the amount of power lost during the commutation process using current matrix converter circuits and commutation methods. This need and others are satisfied by the invention which is directed to matrix converter circuits and commutation methods.